The term “phosphor” as used herein means a substance which absorbs light of a certain wavelength to excite electrons and emits light (fluorescence, phosphorescence) when the electrons thus excited fall back to the ground state.
As the phosphors, the followings are known:
(1) inorganic phosphors composed mainly of an oxide, a nitride, a sulfide, or the like and doped with ions serving as an emission center,
(2) organic phosphors such as rare earth complexes, and
(3) carbon phosphors such as carbon nanoparticles and graphene nanosheets.
Of these, graphene-based carbon phosphors are characterized by that they are excellent in electrical properties, thermal properties, and mechanical properties and in addition, are chemically stable.
There have conventionally been a variety of proposals on such phosphors made of a graphene-based material.
For example, Non-patent Document 1 discloses graphene quantum dots (GQDs) available by:
(1) using, as a starting raw material, micrometer-sized rippled graphene nanosheets (GSs) available by thermal reduction of graphene oxide (GO),
(2) oxidizing the GSs in highly concentrated H2SO4 and HNO3 to introduce, at the edge and on the basal plane, an oxygen-containing functional group such as C═O/COOH, OH, and C—O—C,
(3) subjecting the oxidized GSs to hydrothermal treatment at 200° C., and
(4) filtering and dialyzing the resulting colloid solution.
This document describes that:
(a) deoxidization occurs due to the hydrothermal treatment and the (002) spacing of the GQDs was reduced and became close to that of bulk graphite,
(b) the size of the GSs decreased dramatically by the hydrothermal treatment and ultrafine GQDs (average diameter: 9.6 nm) can be isolated by a dialysis process,
(c) the oxidized GSs show no detectable photoluminescence (PL) behavior, while the GQDs emit bright blue luminescence even in neutral media,
(d) the GQDs shows a PL spectrum with a strong peak at 430 nm on excitation at the absorption band of 320 m, and
(e) The PL quantum yield of the GQDs is 6.9%, comparable with those of luminescent carbon nanoparticles.
Non-patent Document 2 discloses GO thin films reduced with hydrazine vapor.
This document describes that:
(a) the PL characteristics of the GO originate from the recombination of electron-hole (e-h) pairs localized within small sp2 carbon clusters embedded within an sp3 matrix,
(b) the absorbance of the GO increases with hydrazine exposure time, consistent with the evolution of oxygen (from ˜39 at % in starting GO to 7˜8 at % in the reduced GO),
(c) the PL peak position of the GO thin films does not undergo a large change when subjected to reduction treatment and is centered around 390 nm, and
(d) while the FL intensity of the GO thin films is weak for as-deposited GO films, short exposure to hydrazine vapor results in a dramatic increase in the PL intensities.
Further, Non-patent Document 3 discloses graphene quantum dots which have been synthesized through solution chemistry, contain 132 conjugated carbon atoms, and at the same time are enclosed in all three dimensions by three solubilizing 2′,4′,6′-trialkyl phenyl groups.
This document describes that:
(a) the graphene is stable in various organic solvents without aggregation,
(b) when this graphene is dispersed in toluene and excited at 510 nm at room temperature, emission peaks appear at 670 nm and 740 nm,
(c) the emission at 740 nm is phosphorescence and the time dependent behavior of it shows a single exponential decay with a time constant of 4 μs at room temperature, and
(d) the emission at 670 nm is fluorescence and the dynamics of it can be fitted with a biexponential decay with time constants of 5.4 and 1.7 ns.
Carbon phosphors using a graphene nanosheet typically emit blue luminescence as described in Non-patent Documents 1 and 2. It is known that some of conventional inorganic phosphors contain a harmful element such as cadmium, but carbon phosphors do not require such a harmful element in order to achieve PL characteristics.
The emission efficiency of carbon phosphors so far reported is, however, 6.9% at the maximum as described in Non-patent Document 1. Further, carbon phosphors prepared by the conventional process have difficulty in controlling the emission wavelength as described in Non-patent Document 2.